1. Field of the Invention
The present invention relates to a system for enabling non-invasive surgery to be performed by localized heating and more particularly to a system for surgery by localized heating guided by magnetic resonance (MR) imaging.
2. Description of Related Art
Conventional Magnetic Resonance (MR) Imaging provides the radiologist with internal views of a subject's anatomy. MR Imaging provides excellent contrast between different tissues and is useful in planning surgical procedures. A tumor in a subject is much more visible in an MR image than as seen in actual surgery because the tumor and normal tissue often look similar in surgery. The tumor can also be obscured by blood during surgery. A view of the heated region is provided with the use of MR temperature sensitive pulse sequences. MR imaging temperature-sensitive pulse sequences are described in U.S. Pat. No. 5,307,812 May 3, 1994 "Heat Surgery System Monitored by Real-Time Magnetic Resonance Profiling" by C. Hardy, H. Cline which describes capturing temperature mapped imaging of a subject. Experiments on animals show that a heated zone above a critical temperature destroys tissue. Heating tissue beyond a critical temperature for a period of time causes necrosis, destruction of tissue.
Tumors have been selectively destroyed in cancer subjects using focused ultrasound heating guided by MR imaging.
MR imaging employs large magnets for creating a homogeneous magnetic field, and gradient coils for altering the magnetic field in a uniform manner in time or space, creating magnetic field gradients. MR imaging also employs radiofrequency (RF) coils for applying an RF field to tissue to be imaged, causing the tissue to resonate and create an MR response signal. The MR response signal is used to construct an image. The degree of homogeneity of the magnetic field and the linearity of a magnetic field gradient over space are important in creating a clear, undistorted image. Interference with the RF field also reduces the quality of the created image.
Recently, there is a desire to create interactive images of internal organs of a patient during surgery. Since magnetic resonance imaging provides great detail in images of soft tissues, it is advantageous to use MR imaging. The best imaging results when surgical equipment does not interfere with the magnetic and RF fields created by the MR imaging equipment.
Many metals are ferromagnetic and are physically pulled toward a magnet. Since the magnetic field employed in MR imaging is large, an amount of magnetic force applied to the equipment can be large. Equipment used near the MR magnet, therefore, should not be made of a ferromagnetic material since a magnetic force would be applied to them causing them to be difficult to manipulate.
Other problems occur with materials in which eddy currents are produced when placed in a variable magnetic field. The eddy currents in these materials, usually electrical conductors, create their own magnetic field which interferes with the fields used for MR imaging. Therefore, materials which exhibit eddy currents, such as aluminum and copper, should not be used within a changing magnetic field.
Additionally, conducting materials disturb and distort the radiofrequency electromagnetic fields necessary for resonance imaging.
The degree of magnetization the material exhibits per applied magnetic field is defined as susceptibility. The susceptibility of a material also affects the homogeneity of the applied magnetic field in a region surrounding the material. This creates large distortions in an MR image near the material.
In U.S. Pat. No. 5,247,935 Sep. 28, 1993 "Magnetic Resonance Guided Focused Ultrasound Surgery" by H. Cline, R. Ettinger, K. Rohling, R. Watkins; and U.S. Pat. No. 5,275,165 Jan. 4, 1994 "Magnetic Resonance Guided Ultrasound Therapy System With Inclined Track to Move Transducers in a Small Vertical Space" by R. Ettinger et al., assigned to the present assignee and hereby incorporated by reference, an ultrasound transducer is positioned within an MR Imaging magnet with the use of hydraulics. These positioners are MR compatible, however, these required hydraulic lines, pumps and motors, and optical position encoders which added considerable complexity and also required considerable amount of maintenance.
Currently, there is a need for a simplified device which can accurately localize heat to selectively kill or destroy tumor tissue without damage to surrounding healthy tissue while allowing MR images to be acquired.